1. Field of the Invention
This invention relates to a field of medical devices, especially the devices used for delivery of drugs. More particularly, it is directed to coatings for drug delivery devices, such as, for instance, drug eluting vascular stents.
2. Description of Related Art
In the field of medical technology, there is frequently a necessity to administer drugs locally. To provide an efficacious concentration to the treatment site, systemic administration of such medication often produces adverse or toxic side effect for the patient. Local delivery is a preferred method in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Thus, local delivery produces fewer side effects and achieves more effective results.
One commonly applied technique for local delivery of the drug is through the use of medicated stents. One method of medicating a stent is with the use of a polymer coating impregnated with the drug.
References describe a variety of polymers which can be used to coat stents. Of particular interest is a copolymer of ethylene and vinyl alcohol, also known as poly(ethylene-co-vinyl alcohol) or EVOH. Poly(ethylene-co-vinyl alcohol) is also known under the trade name EVAL and is distributed commercially by Aldrich Chemical Company of Milwaukee, Wis. EVAL is also manufactured by EVAL Company of America of Lisle, Ill.
EVAL is a product of hydrolysis of ethylene-vinyl acetate copolymers. Those having ordinary skill in the art of polymer chemistry will understand that EVAL may also be a terpolymer and may include up to 5% (molar) of units derived from styrene, propylene and other suitable unsaturated monomers. EVAL possesses a desirable impermeability to oxygen, bio- and blood-compatibility. EVAL is at least somewhat hydrophobic and thus is somewhat insensitive to moisture.
While EVAL has been shown to be a very inert and biocompatible polymer which is quite suitable for use with medical vascular devices, some of its properties can be improved. In particular, EVAL, due to a high concentration of hydroxyl groups in the vinyl component-derived units of the macromolecule, has strong interchain hydrogen bonding, which makes the polymer initially hard to dissolve in an organic solvent.
Accordingly, EVAL's solubility in organic solvents is limited. At the same time, these hydroxyl groups are responsible for insufficient water resistance, and in many applications EVAL does absorb more water than desired.
EVAL also has a high degree of crystallinity, due to the presence of the units of the macromolecule derived from the ethylene component, and a limited ability to fully control the release of drugs. EVAL's limited ability to fully control the release rate of some drugs below a certain molecular size stems from an insufficient degree of hydrophobicity of EVAL. This leads to a level of water absorption that causes the polymer to swell, increasing the polymer's porosity, and the diffusivity of the drug.
An improvement over EVAL is desired, so that the polymer forming the stent coating has a higher degree of hydrophobicity and a lower degree of crystallinity as compared to EVAL.
In view of the foregoing, it is very desirable to have alternative polymeric materials suitable for the use with various medical devices, particularly, with stents for controlled drug delivery. These polymeric materials should be bio- and blood-compatible, at least partially impermeable to oxygen, melt-processable, have reduced crystallinity, high hydrophobicity, high tensile strength and flexibility, ability to provide slower drug release rates, and be soluble in organic solvents.
The present invention provides a number of such polymers according to the following description.